1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and to an electronic device having the semiconductor device.
2. Description of the Related Art
With high integration of large scale integration (LSI), miniaturization of each element (e.g., transistors) constituting LSI is required. As the size of elements is reduced to miniaturize transistors, a problem called “short channel effect” stands out. A short channel effect results in a drop in threshold voltage or an increase in leakage current, so that unfortunately, reliability of the elements declines.
As measures to suppress the short channel effect, thinning of a semiconductor film which functions as an active layer and a gate insulating film has been under consideration. If a semiconductor film and a gate insulating film are thinned, low contact resistance is required between a metal wiring and the semiconductor film or in an impurity region of the semiconductor film. Therefore, a technique to lower contact resistance or resistance of an impurity region (a source region and a drain region) by forming a silicide film in a semiconductor film has been employed in the semiconductor field. With low resistance in a semiconductor film, an on-current is increased in a semiconductor device, and thus a semiconductor device with excellent characteristics can be manufactured.
In order to silicide a semiconductor film, a method is generally taken in which a metal film is formed on a semiconductor film and heat treatment is performed to make the films react with each other, so that a silicide film is formed at an interface between the semiconductor film and the metal film. However, a thermal process has a problem in that a degree of freedom of selecting a substrate is lowered depending on the heat resistance of the substrate. Further, it is difficult to form a silicide film with high quality by heat treatment at a low temperature (e.g., 450 to 750° C.). In view of the above problem, a technique is attempted in which laser light irradiation is performed with a semiconductor film and a metal film in contact with each other, to make the films react with each other at a high temperature in a short time, so that a silicide film with high quality is formed without thermal damage to a substrate (for example, see Reference 1: Japanese Published Patent Application No. 2000-277750).
However, when a silicide film was provided, using the above laser light irradiation, for a miniaturized structure, i.e., a structure in which a semiconductor film and a gate insulating film are thinned, a problem was caused in which the semiconductor film in a region overlapping with a gate electrode tends to be lost. FIGS. 6A and 6B show transmission electron microscopy (TEM) photographs of a thin film transistor (hereinafter, referred to as a TFT) in which a thin semiconductor film formed over a glass substrate is irradiated with laser light. FIG. 6B is an enlarged photograph of a part of FIG. 6A. In FIGS. 6A and 6B, in the observed TFT, a gate insulating film and a semiconductor film in a region overlapping with a gate electrode are lost, as designated by a dotted circle.
Now a process of manufacturing the TFT in FIGS. 6A and 6B is presented below. First, a silicon oxynitride film which functions as a base insulating film 602 was formed with a thickness of 100 nm over a glass substrate 601, and an island-like semiconductor film 603 was formed with a thickness of 25 nm over the base insulating film. Then, in the following order, a gate insulating film with a thickness of 5 nm was formed over the island-like semiconductor film 603, and a gate electrode 605 having a stacked-layer structure of tantalum nitride with a thickness of 30 nm and tungsten with a thickness of 130 nm was formed over the island-like semiconductor film with the gate insulating film interposed therebetween. After that, a silicon oxynitride film was formed so as to cover the gate electrode and the silicon oxynitride film was etched, so that sidewall insulating layers 606 were formed at side surfaces of the gate electrode.
Next, an impurity element (phosphorus in this case) was introduced in a self-aligned manner using the gate electrode 605 and the sidewall insulating layers 606 as masks, so that a pair of impurity regions were formed in the island-like semiconductor film 603. Then, the entire substrate was irradiated with an excimer laser. FIGS. 6A and 6B are TEM photographs of a cross section of the TFT observed after the irradiation with the excimer laser.
As described above, in the TFT shown in FIGS. 6A and 6B, the gate insulating film and the semiconductor film in a region overlapping with the gate electrode were lost. The present inventors ascribe the phenomenon in FIGS. 6A and 6B to the following cause: when the metal, which is the gate electrode, absorbs the laser light and generates heat, a temperature of the semiconductor film is thought to cross a boiling point because the semiconductor film, which is thinned to have smaller thermal capacity, is heated indirectly. Not only the semiconductor film of a source region and a drain region, but the gate electrode was also heated in the laser light irradiation. A transistor with a miniaturized structure has small thermal capacity due to the thinned semiconductor film. Further, the semiconductor film under a gate electrode is also heated by heat conducted from the gate electrode because a gate insulating film is also thinned.